With reference to FIGS. 1-2, it is known in the automotive industry to use a heat-staking process to join automotive parts. In one known application, a plurality of first thermoplastic components, commonly referred to as heat-stakes 64 may be provided, and a second automotive part 62 may comprise a plurality of second thermoplastic components, commonly referred to as a polymeric base member 66, defining a receiving hole 68 for each heat-stake 64. The receiving holes 68 may be located in corresponding locations to the heat-stakes of the first automotive part 60. The heat-stakes 64 may be inserted into the receiving holes 68 of the corresponding polymeric base members 66 and a staking device (not shown) may be used to heat-treat a leading end 65 of each heat-stake 64. This use of the staking device results in the deformation of the heated leading end of the heat-stake such that a “mushroom cap” 67 may be formed. The mushroom cap 67 of each heat-stake 64 commonly covers the corresponding receiving hole 68 of the polymeric base member 66. This resulting overlap results in the mechanical coupling of the first and second automotive parts 60, 62, It is known that the resulting retention force of any heat-staking process may be dependent upon the amount of surface area of the polymeric base member 66 contacted by the heat-stake's mushroom cap 67 as well as the composition states of the polymeric base member 66 and heat-stake 64. By “retention force” it is meant the force required to separate, break, or undo the mechanical coupling created by the heat-staking process.
Although many known methods of heat-staking work well for their intended purposes, various disadvantages exist. One disadvantage relates to the fastening surface of the polymeric base member available to contact the mushroom cap of the heat-stake. Commonly the fastening surface available is relatively small. Additionally, the process of heating the leading end of the heat-stake may further reduce the amount of fastening area. When a polymeric base member is comprised of a softer composition than that of the heat-stake, the small amount of surface area available for the heat-stake's mushroom head to contact may be insufficient to achieve a high retention force. Often, the heat from the heat-staking process tends to melt some of the polymeric base member's softer material. This melting may create a loose condition that may further reduce retention force.
What is needed, therefore, is a heat-staking process that increases retention force by increasing the amount of fastening area available to contact with the heat-stakes' mushroom cap while also increasing the retention characteristics of the polymeric base member.